Field of the Invention
The present invention is in the technical field of magnetic resonance imaging, and in particular concerns a three-dimensional magnetic resonance imaging method and apparatus.
Description of the Prior Art
Magnetic resonance imaging (MRI) is an imaging modality in which the phenomenon of magnetic resonance is utilized for the purpose of imaging. The basic principles of magnetic resonance are as follows. When an atomic nucleus contains a single proton, as is the case for the nuclei of the hydrogen atoms that are present throughout the human body, this proton exhibits spin and resembles a small magnet (dipole). The spin axes of these small magnets lack a definite coherence, and when an external magnetic field is applied, the small magnets will be rearranged according to the magnetic force lines of the external magnetic field; specifically, they will align in two directions, either parallel or anti-parallel to the magnetic force lines of the external magnetic field. The direction parallel to the magnetic force lines of the external magnetic field is called the positive longitudinal axis, while the direction anti-parallel to the magnetic force lines of the external magnetic field is called the negative longitudinal axis. The atomic nuclei have only a longitudinal magnetization component, which has both a direction and a magnitude. A radio frequency (RF) pulse of a specific frequency is used to excite the atomic nuclei in the external magnetic field, such that their spin axes deviate from the positive longitudinal axis or negative longitudinal axis, and resonance occurs—this is the phenomenon of magnetic resonance. Once the spin axes of the excited atomic nuclei have deviated from the positive or negative longitudinal axis, the atomic nuclei have a transverse magnetization component.
After emission of the RF pulse has ended, the excited atomic nuclei emit an echo signal, gradually releasing the absorbed energy in the form of electromagnetic waves, such that their phases and energy levels both return to the pre-excitation state. An image can be reconstructed by subjecting the echo signal emitted by atomic nuclei to further processing, such as spatial encoding.
In conventional three-dimensional (3D) MRI scanning methods, the parallel encoding is done in two encoding directions (i.e. two-dimensional parallel imaging). Thus, sensitivity variation in two encoding directions can be used to reconstruct an image, e.g. the 2D SENSE undersampling method and the 2D GRAPPA undersampling method. It has been demonstrated in practice that these two undersampling methods can significantly increase the quality of the reconstructed image and speed up image reconstruction. However, these two undersampling methods need sufficient sensitivity variation in the two encoding directions in order to successfully reconstruct an image, and for this reason, the two undersampling methods are significantly reliant upon the distribution of the coils. In addition, the standard 2D SENSE and 2D GRAPPA undersampling methods employ a rectangular undersampling model, which is implemented in each direction by a simple integer sampling reduction.
FIG. 1A is a schematic diagram of a two-dimensional (2D) SENSE undersampling method model for k-space data according to the prior art; FIG. 1B is an image reconstructed according to the undersampling model of FIG. 1A. FIG. 2A is a schematic diagram of a 2D GRAPPA undersampling method model for k-space data according to the prior art; FIG. 2B is an image reconstructed according to the undersampling model of FIG. 2A. As FIG. 1A shows, in a 2D SENSE undersampling method for k-space data according to the prior art, k-space data are undersampled in the ky direction, i.e. every other datum is read in the ky direction (a round dot represents a datum that is read). As FIG. 1B shows, if undersampling is performed according to the model shown in FIG. 1A (i.e. undersampling in the ky direction), artifacts will appear in the image in the ky direction. By the same principle, as FIG. 2A shows, in a 2D GRAPPA undersampling method for k-space data according to the prior art, k-space data are undersampled in the kz direction, i.e. every second datum is read in the kz direction (a round dot represents a datum that is read). As FIG. 2B shows, if undersampling is performed according to the model shown in FIG. 2A (i.e. undersampling in the kz direction), artifacts will appear in the image in the kz direction.
In summary, artifacts will be produced if a standard 2D SENSE undersampling method or 2D GRAPPA undersampling method is used for image reconstruction in a 3D dual-echo or multi-echo scan sequence according to the prior art.